The present invention relates to a thermal protection circuit comprising two electrical connection terminals for a device to be protected, comprising a temperature-dependent switch, which comprises a temperature-dependent switching mechanism, two stationary contacts which are connected to the connection terminals, and a current transfer element, which current transfer element is arranged on the switching mechanism, is moved by the switching mechanism and comprises two counter contacts, which counter contacts are electrically connected to one another, are in temperature-dependent bearing contact with the two stationary contacts and thereby connect said stationary contacts electrically conductively to one another.
A temperature-dependent switch which can be used in the thermal protection circuit is known from DE 26 44 411 C2.
The known switch has a housing with a cup-like lower part, into which a temperature-dependent switching mechanism is inserted. The lower part is closed by an upper part, which is held on the lower part by the upstanding rim of the lower part. The lower part can be manufactured from metal or insulating material, while the upper part consists of insulating material.
Two contact rivets, whose inner heads act as stationary contacts for the switching mechanism, rest in the upper part. The rivet shafts protrude outwards through through-openings in the upper part and merge there with outer heads, which are used for the external terminal connection of the known switch. Connecting litz wires can be soldered directly to these outer heads, wherein it is also known to hold angular contacts on the outer heads, to which angular contacts connecting litz wires are soldered or crimped.
The switching mechanism bears a current transfer element in the form of a contact bridge, two counter contacts being provided on the upper side of said contact bridge, which counter contacts are electrically connected to one another via the contact bridge, are brought into bearing contact with the two stationary contacts, depending on the temperature, and then electrically connect said stationary contacts to one another.
The temperature-dependent switching mechanism has a bimetallic snap-action disc and a spring snap-action disc, through which discs a pin passes centrally which bears the contact bridge. The spring snap-action disc is fixed circumferentially in the housing, while the bimetallic snap-action disc is supported on a shoulder of the lower part or on the rim of the spring snap-action disc, depending on the temperature, and in the process either enables the bearing contact of the contact bridge on the two stationary contacts or else lifts the contact bridge off from the stationary contacts, with the result that the electrical connection between the external terminals is interrupted.
This temperature-dependent switch is used in a known manner to protect electrical devices from overheating. For this, the switch is connected electrically in series with the device to be protected and the AC supply voltage thereof and is arranged mechanically on the device in such a way that it is in thermal contact therewith.
Below the response temperature of the bimetallic snap-action disc, the contact bridge bears against the two stationary contacts, with the result that the circuit is closed and the load current of the device to be protected flows via the switch. If the temperature increases beyond a permissible value, the bimetallic snap-action disc lifts off the contact bridge from the stationary contacts, counter to the actuating force of the spring snap-action disc, as a result of which the switch is opened and the load current of the device to be protected is interrupted.
The now de-energized device can then cool down again. In the process, the switch which is thermally coupled to the device also cools down again and then automatically closes again.
Owing to the dimensioning of the contact bridge, the known switch is capable of conducting much higher operating currents in comparison with other temperature-dependent switches in which the load current of the device to be protected flows directly via the bimetallic snap-action disc or a spring snap-action disc associated therewith, with the result that said switch can be used for protecting larger electrical devices with a high power consumption.
As already mentioned, the known switch automatically switches on again after cooling down of the device protected thereby. While such a switching response can be entirely expedient for protecting a hairdryer, for example, overall this is not desirable where the device to be protected should not automatically switch on again once it has been switched off in order to avoid damage. This applies, for example, to electric motors which are used as drive assemblies.
DE 198 27 113 C2 therefore proposes providing a so-called self-holding resistor, which is electrically in parallel with the external terminals. The self-holding resistor is electrically in series with the device to be protected when the switch is open, with now only a nonhazardous residual current flowing through said device owing to the resistance value of the self-holding resistor. This residual current is sufficient, however, for heating the self-holding resistor to such an extent that it emits heat which keeps the bimetallic snap-action disc above its switching temperature.
The switch known from DE 198 27 113 C2 can also be equipped with a current-dependent switching function, for which purpose a heating resistor is provided, which is connected permanently in series with the external terminals. The load current of the device to be protected therefore flows constantly through this heating resistor, which can be dimensioned such that, when a specific load current intensity is exceeded, it ensures that the bimetallic snap-action disc is heated to a temperature above its response temperature, with the result that the switch already opens in the event of an increased load current before the device to be protected has been heated to an impermissible extent.
Such switches have proven reliable for everyday use. They are used in particular for the protection of electrical devices with a high power consumption because they can conduct high currents via the contact bridge. When such switches do not open at the zero crossing of the AC supply voltage, arcs form between the stationary contacts and the counter contacts in the event of the contact bridge being lifted off from the stationary contacts, and the voltage drop across the switch is reduced to the arc voltage. The voltage drop remains at this level until the applied AC supply voltage changes polarity, i.e. reaches its next zero crossing. Then, the arcs are quenched and the switch is reliably opened.
In the conventional application cases of the known switch with a high switching power, a load current with a high current intensity needs to be interrupted, which means that strong arcs form which in turn results in contact erosion and therefore, as a consequence, long term in a change in the geometry of the switching areas and often also in impairment of the switching response.
In the case of uncontrolled flashover in the interior of the switch, arcs can even cause damage to the bimetallic snap-action disc. In addition, arcs can result in the switching areas on the stationary contacts and the counter contacts sticking together, so to speak, and the contact bridge not detaching or no longer detaching quickly enough from the stationary contacts.
These problems are increased with the number of switching cycles even more, with the result that the switching response of the known switch is impaired over the course of time. Against this background, the life period, i.e. the number of permissible switching cycles of the known switches, is limited, wherein the life period is also dependent on the switching power, i.e. the current intensity of the switched currents.
Switches of the generic type by the applicant have, for example, on an AC supply voltage of 250 volts a conventional life of 10 000 switching cycles given a load current of 10 amperes and 2000 switching cycles given a load current of 25 amperes.
Against this background, there is a need for temperature-dependent switches with an increased switching power and an extended life period.
It is known that the formation of arcs cannot be avoided, but that the damage caused by these arcs can be reduced and retarded by corresponding formation of the switching areas. For this, particular geometries and materials need to be used for the contacts, what makes the design of the known switches complex and cost-intensive.
In order to protect the bimetallic snap-action disc from damage as a result of jumping arcs, in addition the design of the known switch is such that the bimetallic snap-action disc is shielded by the spring snap-action disc and the contact bridge with respect to arcs which are produced during opening of the switch.
Although these protective measures have proven successful overall, they cannot wholly prevent the damage associated with the formation of arcs, which limits the life period and switching power of the known switches to the above-mentioned values which are conventional in particular in the case of switches by the applicant.
In addition, a disadvantage consists in that the described measures are complex and cost-intensive.
In connection with relays and contactors, it is known that arcs can be influenced by electromagnetic fields and can be quenched by capacitive and inductive components in the circuit. Furthermore, it is known to guide an arc occurring in contactors by means of so-called permanent magnet blowout such that the arc is quenched quickly.
Further, DE 31 32 338 A1 discloses connecting a controllable semiconductor valve, for example a triac, in parallel with a contactor comprising two fixed contacts and a linearly moveable contact bridge by virtue of the current terminals of said semiconductor valve being connected to the fixed contacts. The control input of the triac is connected to a terminal at the contact bridge via a series resistor and a flexible line, which leads into the interior of the contactor, which terminal is positioned between the contact points with the fixed contacts.
When the contactor is closed, the voltage drop across the contact points needs to be so low that no effective control current for the triac is formed between the control terminal and its reference terminal, which corresponds to one of the two current terminals. The triac is then not conducting, i.e. remains de-energized.
If the contactor opens as a result of external driving, two arcs are produced which must result in such a high arc voltage for a sufficient time span that the contact bridge to the reference terminal has a sufficient potential difference until a control current flows through the series resistor which can trigger the triac. Once the triac has been triggered, i.e. is conducting, it takes up the load current flowing through the contactor, whereupon the arcs are quenched.
By virtue of the rapid electromagnetic actuation of the contact bridge, said contact bridge moves sufficiently far away from the fixed contacts so quickly that renewed triggering of the triac cannot take place once the load current has been interrupted at the zero crossing of the AC supply voltage.
This method therefore has three critical conditions. The voltage drop across the contact points should not be too great when the switch is closed and should not be too low for a specific period of time when the switch is open. In addition, the interrupting speed should be so great that the triac is not triggered again. In addition, it is at least problematic in design terms that a flexible line needs to be guided into the interior of the contactor.
DE 2 253 975 A discloses a circuit wherein an arc forming on opening of a temperature-dependent switch in an AC circuit is quenched by a triac connected in parallel to said switch. The temperature-dependent switch used is a change-over switch having a central contact, which central contact is connected in temperature-dependent fashion either to a main contact arranged in a load circuit of the load, or to an auxiliary contact permanently connected to the control input of the triac. When the auxiliary contact is closed, a permanent residual current is flowing leading to power loss.